Vino
When I have used natural casings for chorizo and small salami in the past, I have had them crease as they dried. I cannot say if it is directly caused by using natural casings, but more probably due to conditions that were not quite right with regard to temperature and humidity.
I have recently started using protien lined fibrous casings at 2�" diameter and although this is my first main batch since changing casings, they appear to be drying out quite evenly. I would also say that the conditions are the best that I have encountered so far with RH at 68%-74% (have to keep the door of the biltong box cracked open though else RH visits the 90%) and temperature ranging from 13�C-18�C depending on what time of day and night I check it.
I am pleased to say I notice I have white mould starting to appear on both the chorizo and the salami lucesse. Also with the larger casings they seem to be drying quite slowly compared to my earlier efforts.
Don't know if any of my findings are relevant to your problem.
Regards, Paul Kribs